This invention relates to light modulation by frustration of total internal reflection. More specifically, this invention relates to a display in which total internal reflection at a surface is permitted or frustrated by moving electrophoretic particles relative to this surface.
The entire disclosures of all patents and patent applications mentioned hereinafter are incorporated herein by reference.
It has long been known that the transmission of light through an optical system can be modulated by causing the light to undergo total internal reflection at a surface within the system, and permitting or frustrating this total internal reflection by moving one or more members relative to the surface. The xe2x80x9cmembersxe2x80x9d moved relative to the surface can be electrophoretic particles suspended in a liquid and moved relative to the surface by an electric field. For example, U.S. Pat. No. 5,317,667, issued May 31, 1994, describes an electrophoretic switch for a light pipe. The light pipe is surrounded by two concentric cylindrical electrodes, the inner electrode being transparent. Between the electrodes is confined an electrophoretic medium comprising a plurality of charged particles in a suspending liquid. When the electrophoretic particles are spaced from the transparent inner electrode, total internal reflection (TIR) of the light passing along the light pipe occurs at this inner electrode, so that the full amount of light continues along the pipe. However, if an electric field is applied between the two electrodes so that the electrophoretic particles form a layer covering the inner electrode, TIR at this electrode is frustrated, and the flow of light along the pipe is substantially reduced or eliminated.
U.S. Pat. No. 6,215,920, issued Apr. 10, 2001 to Whitehead et al., describes a conceptually similar system (see FIG. 3 of this patent) in which TIR occurs at the interface between a solid light-transmitting member and an electrophoretic medium. The light transmitting member has a series of parallel V-shaped grooves or channels having 90xc2x0 internal angles and having surfaces covered with a transparent electrode material. The opposed electrode has the form of a flat plate on the opposed side of a cavity within which the electrophoretic medium is confined. When the electrophoretic particles do not cover the surfaces of the channels, light enters through a planar surface of the light-transmitting member remote from the channels, strikes the surfaces of the channels, where it undergoes two TIR""s, and is reflected back through the surface by which it entered. However, by applying an appropriate voltage between the electrodes, the electrophoretic particles are moved to form a layer plating the surfaces of the channels and frustrating the TIR""s. Thus the apparatus acts as a light modulator. Mossman et al., xe2x80x9cNew Reflective Color Display Technique Based on Total Internal Reflection and Subtractive Color Filteringxe2x80x9d, SID 01 Digest, page 1054 (Society for Information Display, June 2001) describes a similar system in which the light-transmissive member includes an array of subtractive color filters to provide a full color display. The same paper also describes the use of a polymeric film adjacent the light-transmitting member, this polymeric film being provided with grooves having an internal angle of 60xc2x0 and running perpendicular to the grooves in the light-transmitting member, in order to concentrate incoming light into the light-transmitting member.
The systems described in the preceding paragraph (hereinafter for convenience referred to as the xe2x80x9cWhitehead systemsxe2x80x9d) have a number of problems, and this invention relates to improvements and modifications of such systems. More specifically, this invention is designed to address the following problems in Whitehead systems:
(a) non-uniform distribution of particles on the surfaces of the channels in the dark state of the system;
(b) settling of the electrophoretic particles under gravity; and
(c) non-uniformity of electric field between the electrodes.
This invention also provides a modified Whitehead system in which the dark state of the device is produced by light scattering or absorption rather than total internal reflection.
The basic Whitehead system, as defined in claim 1 of the aforementioned U.S. Pat. No. 6,215,920 is an image display device comprising:
(a) a reflective sheet having a prismatic inward surface and an opposed outward surface;
(b) an electrophoretic medium contacting the prismatic surface;
(c) a plurality of particles suspended in the electrophoretic medium; and
(d) means for applying a voltage across the electrophoretic medium to selectively move the particles closely adjacent the prismatic surface to frustrate total internal reflection at the prismatic surface of light rays passing through the reflective sheet, the electrophoretic medium having an index of refraction sufficiently smaller than the index of refraction of the reflective sheet that most of the light passing through the reflective sheet undergoes total internal reflection at the prismatic surface when the particles are not closely adjacent the prismatic surface.
(For the avoidance of confusion, it should be noted that there is a serious error in the aforementioned claim 1 as printed. The printed claim states that the reflective sheet has an index of refraction n1, the electrophoretic medium has an index of refraction n2 and that n2 is sufficiently larger than n1 to produce the specified TIR. This must be wrong, since TIR occurs when light attempts to emerge from a medium of higher refractive index into one of lower refractive index. Furthermore, according to claim 2 of the same patent, the preferred range for n1 is 2.1 to 2.4, and the preferred range for n2 is 1.25 to 1.27. Accordingly, the above definition of the basic Whitehead system specifies that the electrophoretic medium has an index of refraction sufficiently smaller than the index of refraction of the reflective sheet to produce the specified TIR.)
In one aspect, this invention provides a Whitehead system in which the means for applying a voltage comprises a first electrode substantially conforming to the shape of the prismatic surface and a second electrode substantially conforming to the shape of the first electrode such that the gap between the first and second electrodes is of substantially constant width. This aspect of the invention may hereinafter be referred to as the xe2x80x9cconforming rear electrodexe2x80x9d embodiment.
In another aspect, this invention provides a Whitehead system in which the electrophoretic medium and the plurality of particles are contained within a plurality of capsules. This aspect of the invention may hereinafter be referred to as the xe2x80x9cencapsulatedxe2x80x9d embodiment.
In another aspect, this invention provides a Whitehead system in which the electrophoretic medium comprises a viscosity modifier. This aspect of the invention may hereinafter be referred to as the xe2x80x9cviscosity modifierxe2x80x9d embodiment.
In another aspect, this invention provides a Whitehead system in which the particles bear a polymer coating. This aspect of the invention may hereinafter be referred to as the xe2x80x9cpolymer coated particlesxe2x80x9d embodiment.
In another aspect, this invention provides a Whitehead system in which the volume fraction of the particles in the electrophoretic medium is at least about 50 percent. This aspect of the invention may hereinafter be referred to as the xe2x80x9chigh volume fractionxe2x80x9d embodiment.
In another aspect, this invention provides a Whitehead system in which at least some of the particles are attached to the reflective sheet by flexible filaments (xe2x80x9ctethersxe2x80x9d). This aspect of the invention may hereinafter be referred to as the xe2x80x9ctethered particlesxe2x80x9d embodiment.
In another aspect, this invention provides a Whitehead system in which the plurality of particles vary in electrophoretic mobility, at least one of the particles having an electrophoretic mobility which is at least twice that of another of the particles. This aspect of the invention may hereinafter be referred to as the xe2x80x9cvarying electrophoretic mobilityxe2x80x9d embodiment.
In another aspect, this invention provides a Whitehead system in which the prismatic surface is provided with an electrode and a layer of a substantially insulating material having a low refractive index covering the electrode. This aspect of the invention may hereinafter be referred to as the xe2x80x9clow refractive index layerxe2x80x9d embodiment.
In another aspect, this invention provides a Whitehead system which comprises a rear support disposed on the opposed side of the electrophoretic medium from the reflective sheet, and a plurality of cross-walls extending from the reflective sheet to the rear support and separating the electrophoretic medium into a plurality of cells isolated from one another by the cross-walls. This aspect of the invention may hereinafter be referred to as the xe2x80x9ccross-wallsxe2x80x9d embodiment.
In another aspect, this invention provides a modified Whitehead system in which the electrophoretic medium comprises first and second phases, the first phase wetting the prismatic surface while the second does not, whereby a layer of the first phase is formed covering and substantially conforming to the prismatic surface, the first phase having an index of refraction sufficiently smaller than the index of refraction of the reflective sheet that most of the light passing through the reflective sheet undergoes total internal reflection at the prismatic surface when the particles are not closely adjacent the prismatic surface. This aspect of the invention may hereinafter be referred to as the xe2x80x9cimmiscible phasesxe2x80x9d embodiment.
Finally, this invention provides a modified Whitehead system which contains the same basic integers as the original Whitehead system but does not rely upon frustration of TIR. This modified system, which may hereinafter be referred to as the xe2x80x9cscattering/absorptionxe2x80x9d embodiment, comprises:
(a) a reflective sheet having a prismatic inward surface and an opposed outward surface;
(b) an electrophoretic medium contacting the prismatic surface;
(c) a plurality of particles suspended in the electrophoretic medium; and
(d) means for applying a voltage across the electrophoretic medium to selectively move the particles closely adjacent the prismatic surface.
The particles comprise at least one light-scattering or light-absorptive center disposed within a light transmissive matrix, whereby, when the particles are disposed closely adjacent the prismatic surface, most of the light passing through the reflective sheet passes into the particles and is scattered or absorbed by the light-scattering or light-absorptive center.